Be very, very careful about combining "business trips" with "flight instruction". Some, not all, but some, FAA Inspectors are very suspicious of any flying that could be concieved as "passenger carrying for hire" without a 135 certificate. No matter how you may think you can justify it, if it falls outside of the "normal" training routine, an "official investigation" of the matter will force you to have a lawyer convince an Inspector or an NTSB Judge that the flight's PRIMARY FUNCTION was flight instruction. Any transportation gain that occured was strictly coincidental and not the purpose of the flight.
And to the matter of 8 hours dual in one day. An Inspector would probably say that you don't have to continue to instruct beyond 8 hours, as in a flight that exceeds the limit because of unknown and unforcast conditions, the flight has to continue to termination, but you can stop the instruction at 8 hours. That is a willful violation of a regulation, plain and simple, black and white.